1. Field of the Invention
The present invention relates to a fixing device comprising a fixing member that is heated by heating means and pressing means for pressing the fixing member, which fixes an unfixed toner image onto a recording body using heat from the heated fixing member and pressure generated when the pressing means and fixing member are pressed together, and an image forming apparatus comprising the fixing device.
2. Description of the Background Art
An image forming apparatus employing an electrophotographic method comprises an image creation process for creating a toner image on the surface of a recording body using toner serving as coloring particles, and a fixing process for fixing the created toner image on the recording body. The toner melts when heated and coagulates when cooled. In the fixing process, the physical properties of the toner are taken into account when heating the toner until the toner melts, whereupon the toner is fixed onto the recording body surface.
A conventional fixing device for fixing a toner image on a recording body surface comprises a heat roller having heating means such as a halogen heater disposed in its interior, and a pressure roller that is pressed against the heat roller by a coil spring or the like to form a nip portion between itself and the heat roller. In this type of fixing device, a recording body carrying a toner image is passed over the surface of the nip portion between the heat roller and pressure roller such that heat and pressure are applied to the toner image, and as a result, the toner image is fixed onto the recording body.
In this type of fixing device, an elastic layer is required on the heat roller surface which contacts the toner on the recording body surface to improve the fixing quality. In this case, however, the elastic layer forms a thermal resistance layer when thermal conduction occurs in an outward direction from the heating means in the interior of the heat roller, and hence there is a limit to the thickness of the elastic layer on the heat roller. As a result, it is difficult to make the elastic layer sufficiently effective.
A fixing device described in Japanese Unexamined Patent Application Publication 2002-49257 is a fixing device capable of solving this problem. The fixing device described in Japanese Unexamined Patent Application Publication 2002-49257 has a fixing belt that performs an endless motion while stretched by a plurality of stretching members and is heated by heating means. A pressure roller serving as pressing means for pressing the fixing belt is also provided, and the plurality of stretching members comprises a heating roller serving as the heating means and a fixing roller that opposes the pressure roller via the fixing belt to form a nip portion. In this fixing device, the fixing belt heated by the heating roller moves endlessly toward the nip portion, and by passing a recording body carrying toner through the nip portion, a toner image on the recording body is fixed by the heat of the fixing belt and the pressure of the nip portion. Since both the heating roller and fixing roller are provided such that the heating function and the pressing function are separated, the fixing roller opposing the toner on the recording body surface via the fixing belt may be provided with a thick elastic layer, and therefore, a high quality color image can be obtained.
In the fixing device described above, comprising the heating roller and the fixing roller, the location in which the fixing belt is wrapped around the heating roller serves as a heating region in which the inner peripheral surface of the fixing belt contacts the heating roller such that heat is transferred from the heating roller to the fixing belt. In this type of fixing device, the heating region for heating the fixing belt is typically made long enough to ensure that the outer peripheral surface of the fixing belt at the downstream end of the heating region in the endless motion direction of the fixing belt can be heated to a suitable temperature for fixing. By making the heating region long enough and heating the fixing belt to an extent at which the outer peripheral surface of the fixing belt reaches a suitable temperature for fixing, the toner on the recording body can be heated sufficiently in the nip portion, and fixing can be performed favorably.
However, when the fixing belt is heated to an extent at which the outer peripheral surface of the fixing belt reaches a suitable temperature for fixing, heating is performed with poor heating efficiency. The reason for this is as follows.
During initial heating of the fixing belt, heat that is transferred from the heating means to the surface (to be referred to hereafter as the heated belt surface) of the fixing belt opposing the heating means propagates through the interior of the fixing belt and reaches the rear surface (to be referred to hereafter as the non-heated belt surface) of the heated belt surface in such a manner that the amount of heat transferred from the heating means to the fixing belt per unit time is substantially constant. When the heat transferred from the heating means propagates to the non-heated belt surface, the temperature of the non-heated belt surface begins to rise such that the temperature difference between the surface temperature of the heated belt surface, which is substantially identical to the temperature of the heating means, and the surface temperature of the non-heated belt surface begins to decrease. When the temperature difference between the heated belt surface and non-heated belt surface decreases, heat is transferred from the heated belt surface to the non-heated belt surface less easily, and the amount of heat transferred from the heating means to the heated belt surface per unit time decreases. As the temperature of the non-heated belt surface increases, the amount of heat transferred from the heating means to the heated belt surface per unit time decreases gradually, leading to a gradual reduction in the heating efficiency of the heating that is performed after the heat transferred from the heating means propagates to the non-heated belt surface.
When the fixing belt is heated continuously until the outer peripheral surface of the fixing belt reaches a suitable temperature for fixing, as in this conventional fixing device, the amount of heat transferred from the heating roller to the fixing belt per unit time decreases as the outside surface of the fixing belt approaches a suitable temperature for fixing, and as a result, the heating efficiency deteriorates.
When heating is performed with poor heating efficiency, the time required for heating increases. When the process speed is determined in accordance with the image creation speed of the image creation process, the fixing belt performs an endless motion at a linear speed corresponding to the image creation speed, and therefore the heating region must be increased in length to secure the required heating time. To secure the required length of the heating region, the diameter of the heating roller must be increased, and as a result, the fixing device must be increased in size.
Technologies relating to the present invention are also disclosed in, e.g. Japanese Patent No. 2,909,499.